Method for the creation of highly lustrous surfaceson aluminum workpieces

ABSTRACT

Disclosed is a method for creating a highly lustrous surface on a workpiece which is made of aluminum or aluminum alloy and has a peak-to-valley height of less than 5 m. Said method comprises the following successive steps: a) the surface is mechanically polished in one or several successive polishing steps by means of a polishing means containing small, hard, sharp-edged particles, whereupon the mechanically polished surface is cleaned of polishing residues; b) a colorless and transparent conversion solution is applied to the mechanically polished, cleaned surface, whereupon the conversion solution is dried so as to form a conversion layer; c) a layer of clear lacquer which can be hardened and is based on inorganic or organic coating materials is applied to the surface that is provided with a conversion layer; d) the clear lacquer that has been applied to the surface is hardened.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of International Application PCT/EP2003/011143 filed on Oct. 9, 2003, now PCT Publication WO 2004/039507, the contents of which are herein wholly incorporated by reference

The present invention relates to a method for the creation of a highly lustrous surface on an aluminum or aluminum-alloy workpiece.

For creation of highly lustrous surfaces on aluminum-material workpieces, it is known that the surfaces can be treated by mechanical polishing followed by chemical or electrolytic lustering. For protection of the lustered surfaces, anodic oxidation of the lustered surfaces is always performed.

In the conventional technique, a workpiece of aluminum material, which may have a peak-to-valley height of less than 5 μm, is usually mechanically polished by felt rolls and a polishing agent. The polishing agent is usually based on a wax or talc component, in which aluminum oxide particles are embedded.

For subsequent lustering, the polished workpiece is dipped in a tank containing aqueous electrolytes and connected to the positive terminal of a voltage source, while the electrolyte or the electrically conductive tank is connected to the negative terminal of the voltage source. Because of the electric current, electrochemical reactions take place at the workpiece surface, causing aluminum and its alloys to pass into solution. Because of the development of a poorly conductive, viscous liquid layer having a high concentration of aluminum salts, primarily the peaks, raised structures and irregularities of the surface are attacked by the electric current and eroded by dissolution. The erosion of the raised structures leads on the whole to smoothing of the surface and to associated improvement of the reflection and luster characteristics of the workpiece.

For example, in the very frequently used phosphoric acid/sulfuric acid method (see, for example, Hübner, Speiser, “Practice of anodic oxidation of aluminum”, Aluminium-Verlag Düsseldorf, 4^(th) Edition 1988), in which very good to satisfactory luster effects can be achieved even for aluminum of relatively poor quality and for most alloys, a solution of concentrated H₂SO₄ (sulfuric acid), concentrated H₃PO₄ (phosphoric acid) and HNO₃ (nitric acid) is used as the electrolyte. Furthermore, the electrolyte contains Al (aluminum) in low concentrations.

If no further precautions are taken, however, the luster effect of chemically or electrolytically lustered surfaces is usually lost rapidly once again due to corrosion. To prevent this, an oxide layer more than one hundred times thicker than the natural oxide layer is formed by anodic oxidation. To a limited extent, the anodically created oxide layer ensures that the lustered surface can withstand mechanical loads and that it is protected against corrosion. In particular, by virtue of the anodic oxide layer, the surface is resistant to chemical corrosion in a pH range of about 5 to about 8.

However, the method known from the prior art for creation of highly lustrous surfaces, wherein the surfaces are polished, chemically or electrolytically lustered and then anodically oxidized, suffers from substantial disadvantages. For example, the luster effect of the lustered surfaces is basically reduced by the oxide layer formed, generally in a thickness of about 5 to 7 μm, during anodic oxidation. As has been further found, the effects of heat already lead at temperatures of above approximately 90° C. to hairline cracks in the very hard, anodic oxide layer, thus impairing the optical appearance. Furthermore, the limited pH range of corrosion resistance of the anodic oxide layer offers only inadequate protection against relatively strong acids and relatively strong alkalis (pH below 5 or above 8), and so surfaces lustered in this way cannot be used for certain applications.

In the economic and ecological respect, the method known in the prior art additionally suffers from the problem that, for lustering, there are used solutions that contain acids (sulfuric acid, phosphoric acid, hydrofluoric acid), thus representing environmental pollution and being expensive to dispose of. It is also expensive to dispose of the hydroxide sludges produced by treatment of these solutions as well as of the resulting rinse water. In addition, very high current densities are used during lustering and anodic oxidation, and so correspondingly large quantities of electrical energy are consumed. Above and beyond this, the necessary cooling of the electrolyte leads to a very high energy burden.

Yet another substantial disadvantage of the conventional lustering methods is that the aluminum alloys used are permitted to have only a limited iron content, since it is basically known that the luster effect of the lustered surfaces decreases with increasing iron content in the aluminum alloy. Thus, to create highly lustrous surfaces, it is generally necessary that the iron content of the aluminum alloy be less than 0.04 weight percent. If duller (not highly lustrous) surfaces will suffice, the permissible iron content can even be as high as 0.08 weight percent. Above an iron content of 0.08 weight percent, however, it is generally no longer possible to achieve a satisfactory luster effect of the lustered surfaces.

T. W. Jelinek, in “Surface treatment of aluminum” (1977), pages 27, 28, 48 to 57, 181 to 183 and 560 presents a compilation of fundamental activities for surface treatment of aluminum. A method according to claim 1 cannot be inferred from that publication.

Galvanotechnik 88 (1997), pages 156 to 176, describes different methods for creation of conversion layers.

U.S. Pat. No. 3,625,737 describes, for application of a transparent silicate coating on an aluminum substrate for production of a reflector, a method in which a polished surface is anodically oxidized.

In contrast, the object of the present invention is to provide, for creation of a highly lustrous surface on an aluminum or aluminum-alloy workpiece, a method with which the aforesaid and further disadvantages of the methods known in the prior art for creation of highly lustrous surfaces can be avoided.

According to one proposal of the invention, this object is achieved by the features of claim 1. Advantageous configurations of the invention are specified by the features of the dependent claims.

According to the invention, there is described, for creation of a highly lustrous surface on an aluminum or aluminum-alloy workpiece, a method which is characterized by the successive steps described hereinafter.

Firstly, an aluminum or aluminum-alloy workpiece having a peak-to-valley height of less than 5 μm is prepared. The workpiece surface must be cleaned and degreased. Cleaning and degreasing can be achieved in the conventional manner with aqueous or organic solutions, for which purpose the workpiece is dipped in appropriate baths, for example, or is sprayed with the cleaning solutions.

During the first step of the inventive method, the surface of the workpiece to be lustered is mechanically treated by polishing the surface in one or more successive polishing stages by means of a polishing agent containing small, hard, sharp-edged particles. If several polishing stages are performed, the particles of the polishing agent used in a subsequent polishing stage always have a smaller particle diameter than the particles of the polishing agent used in a preceding polishing stage. In any case, the particles of the polishing agent of the last polishing stage, which can also be the single polishing stage, have an average particle diameter of smaller than 1 μm, in order to ensure that a sufficiently smooth surface is created. In the case of several polishing stages, the particles of the polishing agent used in the first polishing stage can have a particle diameter of, for example, approximately 10 μm.

As polishing agent there is preferably used a paste containing particles of diamond, silicon carbide (SiC) or corundum (Al₂O₃). As an example, a polishing paste containing diamond particles is prepared in the form of a dispersion of diamond powder in alcoholic solution.

The mechanical polishing of the surface of the workpiece to be treated can be achieved by means of felt disks or fibrous nonwoven pads, which for this purpose are impregnated with the alcoholic diamond dispersion.

As an example, the polishing operation can comprise two polishing stages, a diamond polishing paste containing diamond particles with an average particle size of smaller than 10 μm being used at first and a diamond polishing paste containing diamond particles with an average particle size of smaller than 1 μm being used thereafter.

After the surface has been polished, it is cleaned in order to remove from the surface the residues and salts left over from polishing, for which purpose there can be used cleaning agents that do not reduce the luster, such as demineralized water.

After polishing and cleaning, a colorless and transparent conversion layer is applied on the mechanically treated surface. For the conversion treatment of the surface, the workpiece is dipped in a conversion solution or sprayed with a conversion solution. Alternatively, the conversion solution can be brushed on or applied with rolls.

The purpose of the conversion layer is mainly to increase the adherency of a subsequent coating substance and simultaneously to achieve a corrosion-inhibiting effect. For this purpose it is essential that the conversion layer be transparent and colorless, in order not to impair the luster effect of the polished surface.

As is known in the prior art, predominantly a chromate coating is used for this purpose in the case of aluminum materials. However, chromating of the surfaces has the very serious disadvantage that the surfaces usually acquire a yellowish/green coloration. Above and beyond this, the chromates and chromic acid compounds used for this purpose, especially in the form of dusts and aerosols, are extremely toxic and therefore hazardous as far as protection of labor and the ecological aspects of the method are concerned. According to the invention, therefore, there is applied onto the lustered surface a chromate-free conversion layer that is transparent and colorless and therefore does not reduce the luster. Preferably the conversion layer contains a titanium/zirconium compound with an organic polymer in aqueous solution.

The conversion layer applied onto the lustered surface is then dried. For this purpose the workpiece can be pulled over into a drying oven maintained at a working temperature of, for example 60° C.

After the conversion layer has dried, the lustered surface is protected against corrosion by applying a layer of a curable clear lacquer based on inorganic or organic coating substances onto the surface provided with a conversion layer. As is standard procedure, this clear lacquer can be curable by heat or UV irradiation.

The clear lacquer is preferably a clear lacquer based on pure acrylate resin, which imparts very good corrosion resistance to the surface protected in this way.

The clear lacquer, especially acrylate lacquer, is preferably applied electrostatically, in the form of a powder lacquer, onto the workpiece surface. For this purpose, the coating powder is electrostatically charged in conventional manner in a spray gun and sprayed by means of compressed air toward the surface of the grounded workpiece to be coated. Even complex workpiece geometries can be thoroughly covered by spraying the powder lacquer. As an alternative, the workpiece surface can be coated with the clear lacquer by vapor deposition.

The clear lacquer must then be cured. As an example, and depending on the clear lacquer used, the workpiece is pulled over into a conventional oven, in which a cross-linking reaction takes place at temperatures of 180 to 220° C. for 10 to 15 minutes. As an alternative, the clear lacquer can be irradiated with UV light to bring about the cross-linking reaction.

It can therefore be stated that, in the inventive method, the degree of luster achieved solely by polishing the workpiece surfaces with the polishing agent containing polishing particles is already much greater than with the combination of polishing, lustering and anodic oxidation known in the prior art. To this extent there is also no need for a post-polishing lustering method in order to increase the degree of luster. Furthermore, the polished surfaces are not anodically oxidized in prior art manner, since this would lead to the disadvantages mentioned in the introduction, but are treated with a conversion layer as an adhesion promoter and by a clear lacquer based on an inorganic or organic compound in order to protect the luster from corrosion.

Since chemical erosion no longer takes place, the iron content of the aluminum alloy has no influence on the luster effect achieved in the inventive method. For example, in contrast to the lustering method known in the prior art, highly lustrous surfaces can be created particularly advantageously by the inventive method even for aluminum alloys having an iron content of higher than 0.04 weight percent. In particular, highly lustrous surfaces can be created even for aluminum alloys having an iron content of higher than 0.08 weight percent.

The inventive method will not be explained in more detail by means of a specific practical example.

PRACTICAL EXAMPLE

An extruded section having a length of 150 mm and a total developed width of 105 mm was degreased with alkali and then polished in two stages using a diamond polishing paste applied on a rotating felt roll. Before polishing, the peak-to-valley height of the degreased material was about 3.5 μm. The diamond polishing paste in the first polishing stage contained diamond particles with an average particle size of 7 μm, while the diamond polishing paste in the second polishing stage had diamond particles with an average particle size of smaller than 1 μm. The surface was then cleaned with alcohol.

The cleaned surface was then subjected to a conversion treatment in a titanium/zirconium-containing polymer-base solution. The conversion layer formed on the surface was colorless and transparent. The conversion layer was dried at 60° C. for 10 minutes in an oven.

After the drying operation, the surface coated with a conversion layer was electrostatically powder-coated with a GMA acrylate powder lacquer in a manual spray booth. The powder lacquer was then cross-linked at 195° C. for 20 minutes in an electric oven.

After the extruded section had cooled, the degree of luster of the treated surface was determined. The surface had a reflection of 95%, measured against the black-mirror standard.

Furthermore, the surface of the treated extruded section was subjected to the following studies:

1) Kesternich Test According to DIN 50018

As the result, it was found that no spotting was perceptible on the surface after 5 cycles of the Kesternich test.

2) Cross-Cut Test According to DIN 53151

As the result, it was found that the cross-cut was rated OK (GT0).

3) Dipping in a Test Solution of pH 13.5 for a Period of 4 Hours at 25° C., Followed by the Kesternich Test.

As the result, it was found that the reflection dropped by 4 percent after the said treatment had been applied and the subsequent Kesternich test (5 cycles) had been performed. 

1. A method for the creation of a highly lustrous surface on an aluminum or aluminum-alloy workpiece having a peak-to-valley height of less than 5 μm, characterized by the following successive steps: a) the surface is mechanically treated by polishing in one or more successive polishing stages by means of a polishing agent containing small, hard, sharp-edged particles, in which polishing stages the particles of the polishing agent used in a subsequent polishing stage have a smaller average particle diameter than the particles of the polishing agent used in a preceding polishing stage, the particles of the polishing agent used in the last polishing stage having an average particle diameter of smaller than 1 μm, after which the mechanically treated surface is cleaned of polishing residues; b) a colorless and transparent conversion solution is applied on the mechanically treated, cleaned surface, after which the conversion solution is dried to form a conversion layer; c) a layer of curable clear lacquer based on inorganic or organic coating substances is applied onto the surface provided with a conversion layer; d) the clear lacquer applied onto the surface is cured.
 2. A method according to claim 1, characterized in that the polishing agent is a paste containing particles of diamond, silicon carbide (SiC) or corundum (Al₂O₃).
 3. A method according to claim 1, characterized in that the conversion layer contains a titanium/zirconium compound with an organic polymer in aqueous solution.
 4. A method according to claim 1, characterized in that the clear lacquer based on organic coating substances is a pure acrylate resin.
 5. A method according to claim 1, characterized in that the clear lacquer is applied electrostatically or by vapor deposition onto the surface.
 6. The use of the method according to claim 1 for the creation of a highly lustrous surface on an aluminum-alloy workpiece, wherein the aluminum alloy has an iron content of higher than 0.04 weight percent.
 7. The use of the method according to claim 1 for the creation of a highly lustrous surface on an aluminum-alloy workpiece, wherein the aluminum alloy has an iron content of higher than 0.08 weight percent. 